1. Field of the Invention
The present invention relates to an apparatus for controlling a pulse width-modulated (hereinafter referred to as PWM) inverter which converts a DC power into an AC power. More specifically, the invention relates to a PWM inverter which, in an asynchronous operation mode of a low output frequency, performs asynchronous operation with a constant cut period irrespective of the change in the output frequency and which, in a synchronous operation of a high output frequency, performs synchronous operation whose cut period changes accompanying the change in the output frequency.
2. Description of the Prior Art
When a PWM inverter is to be shifted from the asynchronous operation to the synchronous operation or from the synchronous operation to the asynchronous operation, it is essential to minimize the phase difference between the modulated wave and the carrier wave to suppress the transient phenomenon.
In order to shift the inverter from the asynchronous operation to the synchronous operation under the condition of a small phase difference, a method has been proposed in Japanese Patent Laid-Open No. 174070/1985 according to which the synchronous carrier waves and the asynchronous carrier waves are produced simultaneously while gradually changing the period of the asynchronous carrier waves, and the inverter is shifted from the asynchronous operation to the synchronous operation when the frequency and phase of these waves are brought into agreement.
According to the prior art, however, both the synchronous carrier waves and the asynchronous carrier waves must be produced at the same time, requiring a complex circuit structure.